This invention relates to an improved process for rendering and drying of materials belonging to a class of organic materials characterized by containing high moisture and high oil or fat levels. Such materials include those of animal origin, such as the flesh, fat, bones, offal (viscera), and blood of fish, poultry, beef and other livestock animals, including those portions of the animals obtained as by-products during the preparation of the animals for use as fresh meat as well as whole animals when they are not used as fresh meat. Such materials also include those of vegetable origin, such as coconut meats, bananas, avocado fruit and other vegetable materials characterized by containing high moisture level and high fat or oil levels, and which are typically rendered to remove moisture in order to obtain the fat or oil.
Some processes for converting renderable materials into usable by-products have been practiced for hundreds of years. At the turn of the century the primary rendering process was "wet rendering." Essentially, wet rendering consists of feeding the renderable material, especially waste animal products, into an agitated tank. Water is added at a ratio of about two parts water to one part renderable material, and then the tank is heated. Sometimes the water is added in the form of live steam, which also serves to agitate the material. As the mixture boils, the oil (also called fat, grease or tallow) melts and floats to the top where it is skimmed off. The water is drained off and the solid residue (often called tankage) is dried for use as animal feed and fertilizer.
In the early part of the twentieth century, the "dry rendering" process was developed. The dry rendering process takes its name from the fact that additional water is not added to the renderable material. Typically, a dry rendering process uses a closed, agitated, jacketed vessel (often referred to as a cooker), which is generally heated indirectly with steam fed through the jacket, U.S. Pat. Nos. 3,682,091 (Bredeson) and 2,673,790 (Illsley) disclose typical cookers. Such a process using a cooker is referred to herein as a "cooker dry rendering" process. The renderable material is placed in the cooker and cooked at about atmospheric pressure until the material is dry. Sometimes at least a portion of the cooking is done under pressure in order to raise the water's boiling point and thereby allow for sterilization of the material by cooking at a high temperature. After cooking is completed, the melted fat is drained away and the dry material discharged. Often the dry, drained solid discharge is fed to a press where additional oil is removed.
The cooker dry rendering process was first developed as a batch process. A charge of material was put into the cooker, then completely cooked and then removed. The process was then repeated with a new charge or batch.
The cooker dry rendering process was improved by the development of various continuous methods. U.S. Pat. Nos. 3,899,301 (Bredeson), 3,673,227 (Keith), 3,506,407 (Keith), 3,471,534 (Jones), and 3,288,825 (Keith) illustrate such continuous cooker dry rendering methods. Such continuous methods usually involve the use of breakers or grinders to reduce the renderable material to pieces of a more manageable and somewhat uniform size. Such pretreatment sometimes also includes an amount of heating. Such continuous methods are characterized by the continuous feeding of the pretreated renderable material into one end of the cooker and its removal from the other end, with its residency time in the cooker being enough to dry the material.
A relatively recently developed dry rendering process can be described as the "slurry evaporation" process. This process generally involves forming a thick, viscous slurry. This slurry is made by reducing the particle size of the renderable material by grinding or the like and mixing the renderable material with a fluid medium, which is preferably oil or fat previously separated from earlier processed renderble material. The slurry is then pumped to a vet still or evaporator where the slurry is heated under subatmospheric pressures to remove the moisture from the slurry. Thereafter, the oil is separated from the solids left in the dewatered slurry, such as by presses or the like. Slurry evaporation may be carried out as either a batch or a continuous process. U.S. Pat. Nos. 4,007,094 (Greenfield et al.). 3,950,230 (Greenfield et al.), 3,917,508 (Greenfield et al.), 3,782,902 (Madsen et al), and 3,529,939 (Mason) are illustrative of some of the slurry evaporation art.
In the slurry evaporation process there are several reasons for adding oil to the renderable material before it is sent to the evaporator. The additional oil makes it easier to grind the raw material. The additional oil helps to make the raw material fluid enough to be handled py pumping. In addition, it has been recognized that additional oil can form a film on the interior surfaces of the evaporator, which serves to improve operation of the evaporator, as is described in U.S. Pat. Nos. 3,898,134 (Greenfield et al.) and 3,529,939 (Mason).
Problems related to control of cooking conditions arise in all rendering processes. The oil or fat deteriorates upon exposure to higher temperatures, especially during long periods of time, thereby resulting in a poor oil product. Therefore, it is desirable to reduce the temperature to which the oil is exposed and to reduce the time the oil is exposed to high temperature. When renderable material is unsufficiently cooked, it is too moist and is difficult to press. But when renderable material is overly cooked, it tends to fall apart and produce fine pieces of solid material, called fines, which are difficult to remove from the oil. Therefore, it is desirable to prevent both over- and under-cooking. Since cooking time varies with, among other things, particle size, moisture content, and oil content, is is easier to control cooking conditions when these variables are controlled and fairly uniform.
One reason that the continuous cooker dry rendering process is an improvement over the batch process is that it provides a continuous discharge of rendered material from the cooker. This discharge may be sampled in order to monitor the temperature, consistency, and other characteristics of the cooked material. This information can be used to adjust the material input, cooking temperature, and other variables of the cooker.
Foaming and boil over is another concern in rendering processes. Essentially, foaming can be described as the formation of an excess of steam-filled bubbles as renderable material is heated. Foaming can interfere with the proper functioning of the rendering apparatus. The lower the pressure, the more likely foaming is to occur, and so vacuum operations are rather susceptible to foaming. Since foaming is a function of, among other things, moisture content, variations in the moisture content of the rendering material make foaming harder to control.
One of the reasons that the slurry evaporator process is an improvement over dry rendering is that evaporators are generally fitted with entrainment separators, carryover chambers, or the like which are of sufficient volume to contain a certain amount of foaming. Since foaming is thereby better controlled, the renderable material may be heated at rather high vacuums.
Another advantage of slurry evaporation processes over the processes which preceded it (such as wet rendering and cooker dry rendering) is the ability to increase energy efficiency by steam savings through what is called multistage or multiple effect evaporators. A simple example illustrates the advantages of a multiple effect evaporator. Consider a rendering system with two evaporators. The renderable material is first sent through evaporator A and then evaporator B. A steam source such as a boiler supplies the heat source for evaporator B. However, the heat source for evaporator A is not a separate boiler, but evaporator B. The hot vapors generated from the renderable material in evaporator B are used to heat the renderable material in evaporator A. Evaporators A and B can be referred to as the first and second stages, respectively, when one is speaking of the flow of renderable materials. Evaporators A and B can be referred to as the second and first effects, respectively, when one is speaking of the flow of steam. It should be noted that the flow of steam is opposite the flow of renderable material, and so, while there are the same number of stages and effects, the numbering of stages and effects start from opposite ends of the system.
Theoretically, steam requirements would be cut in half by such an arrangement of evaporators A and B. Several stages may be used. For example, a three stage system would ideally require only one-third the steam, a four stage system only one-fourth the steam, and so on. However, the temperature of the slurry must be raised in going from one stage to next, and the heat so used (often referred to as sensible heat) is not available to evaporate the water in the slurry. In addition, there are various inefficiencies and losses, such as transmission losses and radiation from the equipment, which further reduce the relative overall improvement in heat requirements. U.S. Pat. No. 4,007,094 (Greenfield) is an example of a multistage system.
In order for multistaging to work, the earlier the effect (later the stage) of the evaporator the higher its operating temperature must be. This is dictated by, among other things, the laws of thermodynamics which state that heat transfer occurs from a higher temperature body to a body of lower temperature, and so the steam must be hotter than the slurry it heats. In addition, the slurry is heated to a temperature above that of the boiling point of the water in it (this difference is sometimes referred to as boiling point rise), and this additional heat serves to help break the attraction between the water and slurry so that the water can be freed. Typically, a temperature difference between stages on the order of 30.degree. to 120.degree. F. is used in efficient slurry evaporation systems. The necessary temperature differences are accomplished in multistage evaporation systems by operating each later effect (earlier stage) at a lower pressure (higher vacuum) then the next earlier effect (later stage).
Similar exploitation of multistage use of steam is difficult in systems using cookers, rather than slurry evaporators. Cookers are often operated at a slight vacuum in order to provide a pressure differential to draw the vapors out of the vessel. Cookers used in batch processes are sometimes evacuated at the end of the batch in order to remove the vapors from the vessel. However, foaming problems make typical cookers ill-suited for continuous operation at low pressures (high vacuums). Therefore, typical cookers are not adaptable to the use of vacuum operation to obtain the required temperature differentials between stages, as in slurry evaporation systems. Theoretically, the necessary temperature differentials could be obtained by operating the later stages at high pressures. But this last alternative is unattractive because of the problems, such as cost, inherent in adapting a series of cookers to high pressure operation. This is particularly unattractive where one wishes to retrofit an existing facility already equipped with cookers not adapted to high pressure operation.
Although slurry evaporation processes have several advantages over cooker dry rendering processes, cookers have not been entirely displaced by slurry evaporators. Slurry evaporation systems are more expensive to build than systems using cookers. Thus, the better energy economy of slurry evaporators may be offset by higher capital investment.
There are also some materials which are more efficiently handled by cookers than slurry evaporators. In all slurry evaporation processes the renderable material must be ground into rather small particles. The handling of bones and other hard materials in a slurry evaporation system has a high energy cost, generally in electricity used to operate grinding machinery. In addition, there are materials, such as hair, feathers, rawhide, and the like, which are troublesome to render in an evaporator because they tend to clog up tubes in the evaporators. Furthermore, slurry evaporation processes typically operate by recirculating the slurry. This results in the recirculation of fines and sludge, thereby presenting oil quality control difficulties.
U.S. Pat. No. 3,632,615 (Mason) discloses cookers and a slurry evaporator used together in a single process. However, the combined use of both the slurry evaporation and dry rendering processes suggested therein presents some difficulties. For example, there is no attempt to reuse hot vapors generated during cooking. This wastes available heat. Furthermore, when an evaporator is used, oil is not pressed from dewatered material until after the material has gone through both the cookers and evaporator, and there is even provision for recycling some oil from the slurry evaporator to the cookers. This compromises oil product quality by promoting a high residency time for the oil and the recycling of fines. In addition, to the extent that the material in the cookers is kept in a liquid state, the surface area of the cooking material is less than that available in an agitated, semi-solid or solid state, as is typical of most cooker operations. This impedes one of the functions of agitation, which is to help to release vaporized moisture from the material being cooked. Other problems with the process disclosed in U.S. Pat. No. 3,632,315 and other prior art will become apparent to one of skill in the art upon study of the improvements made by our invention.